Insulation calculations

[Ashful]

I'm converting an old'ish (1850's?) carriage barn on my property to heated and insulated shop and storage space. Heating will be via dual mini-split. Building envelope is 1200 sq.ft. heated plus 300 sq.ft. unheated space. Spray foam is high on the list of insulating options, given style of construction.

I'm trying to make choices on how thick to go with insulation (eg. R7 per inch in closed cell foam) on walls and ceilings, and whether to make unheated space part of insulated envelope, and even what garage doors to buy (R9's that look nice or R16's that don't). I'd like to find a set of formulae I could use in determining how much to spend on the insulation, in various areas (doors, walls, roof), versus how much those extra BTU's will cost me in heating. Any good on-line calculators or sites describing trade-offs, that you can recommend?

Thanks!

 

[Bret Chase]

how is the barn framed?

 

[Ashful]

Original frame is 8" x 6" sills with 6" x 6" posts and beams, and 3" knee braces. Standard 19th century timber framing, at least for our locale. I failed to notice what type of mill the timber was sawn on, but now that you ask, I should try to do so before it's all covered up! Being a barn, things are somewhat dirty and it has been wet at some point in it's past... the saw mill marks are not abundantly apparent.

 

[Ashful]

Before:



Now:

 

[midwestcoast]

Did you try googling Heat Load Calculators that HVAC folks use?

 

[Circus]

Insulation has a deminishing return as thickness go's up. From R1 to R19 savings can be 500% but from R19 to R38 savings might be 5%. Investigate fiberglass batts for pole buildings. Maybe house wrap against the inside of the exterior wall prior to insulating to prevent drafts circumventing the insulation.
There are insulating calculators and ideas here. http://www.builditsolar.com/ Hit the references tab.

 

[Bret Chase]

Original frame is 8" x 6" sills with 6" x 6" posts and beams, and 3" knee braces. Standard 19th century timber framing, at least for our locale. I failed to notice what type of mill the timber was sawn on, but now that you ask, I should try to do so before it's all covered up! Being a barn, things are somewhat dirty and it has been wet at some point in it's past... the saw mill marks are not abundantly apparent.

Here we'd call that "3/4 framed"... it's framed the same as my barn... timber framed walls with conventional or raftered roof instead of king rafter and purlin.

if it was me... I'd not spray and stuff it all with XPS or polyiso rigid. I would not, under any circumstances spray the ceiling, because that metal roofing will have to be replaced at some point, and it does need to be able to move.

In a former life, my occupation involved installing/removing/replacing metal roofing... measured in acres. I've had to pull off a couple of roofs that were foamed in place.... it easily took 3 to 4 times as long to replace... and the all the advantages of spraying were destroyed.

I would put 4" in the wall and 6" in the roof.

my uncles has the same JD855.. nice little tractor

 

[brian89gp]

I like the HVAC-calc software.

 

[Ashful]

Thanks, guys! Never heard the "3/4 framed" term Bret, but it makes sense. Not many king-post houses around here, but my family did own one for generations (1692 - 1993). I only see timbered roof framing in the 18th century (1700's) houses, and almost all of them were redone with rafters in the 19th century. This barn is almost certainly mid 19th century, not earlier.

I also have my reservations about spray foaming. I'd never use it on a historical house, and even dislike the idea of spraying the timber on this barn, for the sake that someone may want to remove my "modern" work generations from now. With regard to the metal roof, I had been given the impression it peels pretty easily off closed-cell foam, but maybe I was mislead there. I do have an old friend who does blown-in cellulose, so I'll have him in to take a look at this. I don't think the construction I have will play well with the strengths and weaknesses of blown-in.

Bats are always an option, but I'd have to put some more thought into how deep to stuff it (sheathing to wallboard distance varies substantially), and the job may be more DIY, as I don't know of any contractors around here who still use bats. I did my last such barn completely myself, but have very little time these days for DIY. Time is much more precious than money, at this point in my life.

 

[Bret Chase]

with a lift or two of sheet foam... that barn could be knocked out pretty fast.

Contractors around here use whatever the architect/engineer calls for, be it bats, sheet foam, dense-pack, cellulose, blown in, etc...

metal roofing does peel off *relatively* easy... but that's not the main problem. the larger problems are having to shave off ALL of the highs left in the spray foam... because unless you manage to get the exact same crew, with the exact same panels, at the exact same temperature... the highs won't line up. also... that seal between the foam and the metal is gone... moisture will crawl up it, and damage the underside of the panels. AT least with sheet foam... if applied to the underside of the roof strapping... will allow air to move underneath the panels and carry moisture away... keeping the panels dry.

like I said... been there, done that.

 

[Firewood Bandit]

I think you would get more bang for your buck by stopping in air infiltration. Tyvek wrap is good stuff as well as sealing possible leak points.

A couple of years ago we had an energy audit done. They had us seal box sill and the tops of the drywall going into the attic.

 

[Warm_in_NH]

If you do want to spray the roof, I would suggest an air chase below the metal. You can do this with 1" strip of wood down the sides of the rafters, put rigid foam to that leading the 1" gap between rigid and "strapping" then spray away.
Allows it to breathe, you get an air tight seal, max r value and you can replace the roof without issues.
A little more labor but worth it, there's a good chance that roof will condensate and eventually ruin the fiberglass, really needs some venting prior to any insulation.

 

[semipro]

Did you try googling Heat Load Calculators that HVAC folks use?

I believe you're thinking of "Manual J".

 

[semipro]

I'd suggest foam board insulation with an attached radiant barrier be installed along the bottoms of the rafters (radiant barrier up) for the roof.
It will be relatively easy to install and likely less expensive and problematic than spray foam.

 

[Ashful]

Follow-up: Spoke with an old friend today, who is a blown-in cellulose contractor. He did not feel cellulose was the best option for my walls, as the studs are spaced off the sheathing. He said he would spray foam the walls. He also said he could do the roof, but again probably not the best option for my circumstances.

He agreed with the comments above (Bret, NH) on not spraying right to the metal roof, and suggested pinning rigid foam board to the underside of the furring strips to which the roof is fastened in each rafter bay, and then spraying to that foam board. He seemed to have several concerns with condensation dripping off the bottom side of the metal roof, no matter what insulation I used, so I guess vapor barrier behind the wall board is key.

Outside will be wrapped, although exact product has not yet been selected.

As mentioned framing in gable end walls is set in 3" from exterior sheathing, so if I use bats I can either go 6" deep, or just allow dead air space between bats and sheathing. That might not be a bad way to go, after all.

Thoughts?

 

[Warm_in_NH]

The "professional" builders that built my house a strapped the rafters then did metal. I have a cold attic with a full vapor barrier between it and living space. The metal condensates in the winter. It'll build frost until it warms up enough to run down the proper vent and eventually drip through my kitchen window trim. Happens twice a winter, unloads about a quart of water, annoying. VERY confusing the first year I owned the house.

Torn on the best way to fix it. Either rigid air chase and spray foam, or pull all the metal, sheath it, strap it, metal and on the inside proper vent and more insulation.

Don't imagine you'll be generating too much moisture in the barn but it's something to keep in mind.

 

[Ashful]

Running equipment in the attached shed, and even occasionally in the barn, will be about the extent of my moisture generation. We won't be cooking out there. Then again, any air infiltration between the metal roof and insulation will bring in moisture from outside.

I trust this guy. He's been insulating higher end custom homes for 35 years, including the 1994 addition to my own house (wasn't mine then). I just wish I got a more detailed plan from him, but we were only talking on the phone, he wasn't here.

 

[woodgeek]

Agree with the friend's plan for walls and roof.

A few rules of thumb:

To get BTU/h, its is simply = Area(sq ft)*DeltaT(°F)/(R-value). To get BTU/yr it is simply Area(sqft)*24(h/day)*HDD/R-value, where HDD is heating degree days for your location....say 4500±500 in Philly burbs. You can add up the different segments of the building (with their estimated areas and r-values) in a SS trivially to get a total, and see how different changes affect the bottom line. The free online calc do just this, and nothing more. The fancier methods are not free.

Air infiltration will increase the heating load in a leaky building by 100 to 150% over the above (making insulation kinda pointless), if the building is 'snug' it might still add 50%, if it is carefully airsealed I would still add 20%.

You can prob neglect BTU losses to the slab in the above, but the floor will be cold.

The math above doesn't like insulating some areas more than others....insulating half at R-11 and half at R-22 is a waste. The exceptions would be heat rising, and the roof being cold from radiation....more in the roof can make sense.

Garage door manufacturers are poorly regulated re their insulation value....most numbers are notoriously inaccurate, with the highest numbers being the biggest cheats. They also leak a LOT of air....look for the doors that you would judge to be most airtight, and if they have 1+ inches of foam, pick those.

 

[Bret Chase]

I forgot about the garage door question. If it was me... I would find your local Overhead Door branch. The doors are not cheap, but they are the real deal. Every project I have worked on since '89 (commercial and residential) that required a garage door, got one from OHD. They are orders of magnitude better than the crap doors you can get from Home Depot or Lowe's..

 

[Ashful]

I'm going to be dropping some substantial coin on the doors, mostly due to cosmetic reasoning, and fitting odd-size openings without framing them down to fit smaller standard doors. I'd love to go with some of the stamped steel doors rated R-16, accurate or not, but they just don't look good on this barn. So, doors will be machined T&G construction, but using polystyrene backed PVC components. They're rated R-9, which ain't great... but like woodgeek said, air infiltration is likely the larger problem. Door installation is being handled by a local door company who seems to be very good, and does a lot of similar heated shops, and weatherstripping is being addressed.

The working plan for the roof is to cut sheets of foam board, exact material not yet selected, to fit between each rafter. These will be stapled to under side of the furring strips to which the metal roof is attached, allowing ventilation between roof and insulation, and preventing the problems mentioned above with roof replacement. We will then spray foam to the foam board and rafters, and cover with OSB for wall board.

Walls, could be bats or sprayed, but due to air infiltration, I'm really leaning toward sprayed. I still haven't seen many opinions on having insulation sprayed against outside sheathing, versus bats on the interior framing. There is up to 11" space between wallboard (new interior framing) and outside sheathing.

Building will be wrapped in house wrap (Tyvek or similar), with vertical furring strips and horizontal wood siding, outside of that. So, we'll have the usual siding, air space, drainage plane, sheathing, insulation, wall board, construction... the only question being location of that insulation and vapor barrier. I do have some concerns in that regard, with closed cell spray foam on sheathing, and a large air space between wall board and spray foam, but I guess it's standard practice...

 

[woodgeek]

The working plan for the roof is to cut sheets of foam board, exact material not yet selected, to fit between each rafter. These will be stapled to under side of the furring strips to which the metal roof is attached, allowing ventilation between roof and insulation, and preventing the problems mentioned above with roof replacement. We will then spray foam to the foam board and rafters, and cover with OSB for wall board.

Walls, could be bats or sprayed, but due to air infiltration, I'm really leaning toward sprayed. I still haven't seen many opinions on having insulation sprayed against outside sheathing, versus bats on the interior framing. There is up to 11" space between wallboard (new interior framing) and outside sheathing.

Sounds like a plan....for cheap foam board, you can buy 'used' from: http://www.insulationdepot.com/ and save money and be 'green'.

That outfit doesn't do small lots, usually trucks a large shipment, but it sounds like you want a lot of foam board, and don't care if its a little scuffed.

Depending on price, it might make sense to go as thick as possible on the board, and then go thin on the spray-foam for airsealing and hitting the rafters. Remember that poly-iso loses R-value in extreme cold....if the used XPS is not much more, I would go for that, esp if you were going thin on the sprayfoam

 

[semipro]

I'm not understanding why both foam board and spray foam are proposed for the roof. Unless things have changed recently, board foam is less expensive per volume and can easily be air sealed when installed using canned foam.
Perhaps the justification is labor costs?

 

[woodgeek]

Cutting and airsealing 14" strips of foamboard is going to be labor intensive (costly), and the rafters still provide a thermal bridge. Just sprayfoaming it will be material intensive (costly), might need a couple passes to get to target R-value (more labor), and result in an unvented roof assembly. On a big job, I suspect a table saw could be set up to rip the boards quickly, cutting them all underwidth slightly so they go right up quickly. A thinner coating of sprayfoam on the rafters (and the joints with the sheets) would be a quick way to airseal the whole thing and defeat the rafter thermal bridges, and minimizing the use of expensive spray foam.

I'm thinking 4" of (used,cheap) XPS sheet + 1" of sprayfoam and 1" of sprayfoam over the rafters = R-25-30 assembly value, and prob cheaper than 5" of sprayfoam on the deck (undesirable) and 1" on the rafters. I think the foamers have tools for shaving the excess of the lower edges off the rafters quickly, to mount the OSB.

Of course, since you are doing an OSB ceiling, you could fit 2" foam sheet, do the OSB, and blow the cavity with cellulose. Might be the cheapest route, though you'd have to have a plan for sealing, like taping the foam joints with each other and to the rafters.

 

[Ashful]

Great ideas, guys. But in this case, up-front cost (within reason) is not a big issue... my free time is. I can pay a guy to sprayfoam this whole joint in a day, and it's a minimal investment of my time. I'm only agreeing to do the foam board so that I can keep the sprayfoam off the underside of the metal roof, and avoid headaches down the road.

I had planned to use thin foam board (eg.1/2") so I can just cut clean with a utility knife, rather than dealing with the mess of ripping heavier sheets on the table saw or bandsaw. I also already have estimates in the work from two spray foam guys, based on doing 3.5" (R25) in the roof, and it's probably easier to just go with what was quoted than re-figure for less foam / more board.

Do you see any issue with using thinner board? Thoughts on foil back (foil facing roof with 1" air gap) vs. non-foil back?

 

[woodgeek]

If you were going to do the foam sheet and OSB yourself, you could still go a little thicker on the foam, cut it easily (I can do 1" easily if I have fresh blades), and have a cellulose guy fill the cavity cheaply. You could use the savings to pay your next light bill, or maybe two. Of course, you would have to tape the foam sheets to the rafters, and that is time, unless you pay the kid nextdoor to do it.

This of course comes down not to money, but if you wanted to avoid a sprayfoam sol'n due personal preference. Also, don't believe the R-7/in stuff, after it ages/outgases for a while, it'll bottom out around R-5. At 3.5" thick and some residual bridging from the rafters, it could be well under R-20 for the assy.

If the cellulose was thicker it could easily get a higher final R-value. If you were going to heat it with a mini, that might be useful if you were near the limit of a single unit.